1. Field of the Invention
This invention relates to an endoscope optical system which is used in an endoscope combined with lasers for irradiating affected parts in a human body with laser light to take medical treatment while observing the affected parts.
2. Description of Related Art
Endoscopes have been widely used for observation diagnoses and medical treatments of stomachs, large intestines, and the like. In particular, medical treatment with laser light is being popularized for which a laser treatment unit is employed to cut out, cut off, or cauterize the affected parts in the human body. As lasers for medical treatment of such laser treatment units, YAG lasers producing light of wavelength 1060 nm have been chiefly used, but in recent years, small-sized, inexpensive semiconductor lasers have come into use. The wavelengths of light emitted from the semiconductor lasers for medical treatment vary from 750 to 1050 nm.
More recently, on the other hand, electronic endoscopes have been used in which a solid-state image sensor (hereinafter referred to as "CCD") is placed at the distal end or eyepiece section of the endoscope to make observations through a monitor TV. The CCD is sensitive to near-infrared light, and thus when the medical treatment is taken by the YAG laser or the semiconductor laser, reflected light in a near-infrared wavelength region from parts to be treated falls on the CCD. Consequently, the problem is encountered that the image plane of observation is too bright to view an object.
Thus, an endoscope optical system is proposed, for example, by Japanese Utility Model Preliminary Publication No. Sho 55-43053, in which filters for eliminating light emitted from the YAG laser or the semiconductor laser (hereinafter referred to as "laser cut filters") are provided in the optical path of the imaging optical system of the endoscope optical system so that a normal image can be displayed even in laser treatment. Most of such laser cut filters are multilayer film interference filters for reflecting light of wavelengths emitted from the laser. It is desired that the multilayer film interference filter reflecting laser light minimizes a transmittance in the wavelength region of the laser light to completely remove the laser light, has the highest possible transmittance in the wavelength region of visible light to secure a bright, normal observation image, and holds the transmittance constant (namely, minimizes a ripple) in the wavelength region of visible light to improve color reproducibility.
In general, the interference filter is such that when the angle of incidence of a ray of light thereon is varied, its optical properties change in such a manner that spectral transmittance characteristics and the ripple in the visible region largely change. Here, the angle of incidence of a ray of light indicates an angle made by the normal of the coating surface of the interference filter with the ray incident on the interference filter. Also, the interference filter has a tendency that if the elimination rate of laser light is increased or the range of wavelengths to be eliminated is extended, its optical properties will be considerably changed as the angle of incidence of the ray increases.
The endoscope combined with the laser treatment unit also has the multilayer interference laser cut filters in the endoscope optical system to cut laser light. However, the use of filters such that the optical properties vary greatly with the angle of incidence of the ray causes the coloring of an observation image to be changed, resulting in degraded color reproducibility. In this case, if the number of film layers of each filter is increased in order to minimize the dependence of the optical properties on the incident angle and cut a wide band of wavelengths, the problem will arise that the surface profile of the laser cut filter is deteriorated because not only is the fabrication of the filter difficult, but also the interference film becomes liable to peel and damage.
In order to solve these problems, for example Japanese Utility Model Preliminary Publication No. Hei 6-63009 proposes that interference films for removing light of wavelengths different from each other are provided on both surfaces of each optical element placed in the optical path of the endoscope optical system. According to this prior art, even where the range of wavelengths cut by each individual interference film filter remains narrow, light of wavelengths corresponding to various kinds of laser light of different wavelengths can easily be removed to secure a good image which is not affected by laser light.
The foregoing prior art, however, does not in any way refer to the position where each laser cut filter is located. If the laser cut filter is placed at the position where the incident angle of a light ray is relatively large, color variation may affect the image. Furthermore, the prior art is such that, for example, two particular kinds of laser light, such as semiconductor laser light with a wavelength of 800 nm and YAG laser light with a wavelength of 1060 nm, can be eliminated, but laser light of other wavelengths cannot be removed.
Wavelengths emitted from the semiconductor laser vary from 750 to 1050 nm, depending upon its type, and this where various semiconductor lasers and YAG lasers are used, it is necessary to remove laser light over a considerably wide range of wavelengths. In this case, the characteristics of the interference films provided on the surfaces of the optical elements to remove light of different wavelengths must be such that laser light having a wide range of wavelengths can be removed.
If the interference films are designed to have the characteristics such that laser light can be removed in a wide range of wavelengths, however, the following problems will be encountered.
In the first place, if each filter is located at the position where the incident angle of the ray becomes large, a change of spectral transmittance characteristics and the ripple in the visible region will increase, and the observation image will have a difference in color between the center and the periphery thereof. The result is that the image plane of observation becomes hard to see and, for example, a change to a morbid state of a patient may escape notice. In the second place, to remove light having a wide range of wavelengths, each filter used must have a thick interference film with an therefor unfavorable surface profile. If the filter is situated at the position where a bean of light is fine, flaws in the surface of the filter will be conspicuous on the image plane, which becomes hard to see. In the third place, where both surfaces of the optical element are coated with the interference films, one surface is first coated, and then followed by the other. This member requires heating when coated, and thus the thermal expansion of the member on heating may cause the primary interference film coating to crack or peeled off.